xfs: introduce attr remove initial states into xfs_attr_set_iter

xfs: introduce attr remove initial states into xfs_attr_set_iter

Source kernel commit: e5d5596a2a1790d8c57938f820aa33e58f90ad0d

We need to merge the add and remove code paths to enable safe
recovery of replace operations. Hoist the initial remove states from
xfs_attr_remove_iter into xfs_attr_set_iter. We will make use of
them in the next patches.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Allison Henderson<allison.henderson@oracle.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Dave Chinner <david@fromorbit.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>